U.S. patent number 6,958,696 [Application Number 10/268,108] was granted by the patent office on 2005-10-25 for transfer function system for determining an identifier on a surface acoustic wave identification tag and method of operating the same.
This patent grant is currently assigned to RF Saw Components, Inc.. Invention is credited to John C. Bellamy, Clinton S. Hartmann.
United States Patent |
6,958,696 |
Hartmann , et al. |
October 25, 2005 |
Transfer function system for determining an identifier on a surface
acoustic wave identification tag and method of operating the
same
Abstract
A transfer function system for determining an identifier on a
surface acoustic wave (SAW) identification tag and a method of
operating the same. In one embodiment the system provides for (1)
generating a radio frequency (RF) interrogation signal that causes
a transducer located on a piezoelectric substrate to produce an
initial acoustic pulse that reflects off of a plurality of
reflectors arranged according to time and phase position on the
substrate to yield response acoustic pulses, the transducer
generating an RF response signal from the response acoustic pulses;
and (2) determining the identifier by decoding the RF response
signal in view of predefined time, phase and amplitude
parameters.
Inventors: |
Hartmann; Clinton S. (Dallas,
TX), Bellamy; John C. (Coppell, TX) |
Assignee: |
RF Saw Components, Inc.
(Richardson, TX)
|
Family
ID: |
32092406 |
Appl.
No.: |
10/268,108 |
Filed: |
October 9, 2002 |
Current U.S.
Class: |
340/572.1;
310/313D; 340/10.1; 340/572.7 |
Current CPC
Class: |
G08B
13/2417 (20130101); G08B 13/2422 (20130101); G08B
13/2431 (20130101); G08B 13/2485 (20130101); G08B
25/007 (20130101); H03H 9/6406 (20130101) |
Current International
Class: |
G08B
13/24 (20060101); H03H 9/00 (20060101); H03H
9/64 (20060101); G08B 013/14 () |
Field of
Search: |
;340/572.1,572.4,572.7,10.1,10.3,10.41,572.2,10.42
;310/313B,313C,313D,313R,313A ;235/454,455 ;342/42,51,43,50 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Trieu; Van T.
Claims
What is claimed is:
1. A system for determining an identifier on a surface acoustic
wave (SAW) identification tag, comprising: generating a radio
frequency (RF) interrogation signal that causes a transducer
located on a piezoelectric substrate to produce an initial acoustic
pulse that reflects off of a plurality of reflectors arranged
according to time and phase position on said substrate to yield
response acoustic pulses, said transducer generating an RF response
signal from said response acoustic pulses; determining said
identifier by decoding said RF response signal in view of
predefined time, phase and amplitude parameters.
2. The system as recited in claim 1 wherein said RF interrogation
signal has a carrier frequency of about 2.44 GHz.
3. The system as recited in claim 1 further comprising a group of
reflector locations on said substrate.
4. The system as recited in claim 3 wherein a plurality of said
groups are located on said substrate.
5. The system as recited in claim 4 wherein fourteen of said groups
are located on said substrate.
6. The system as recited in claim 5 wherein up to 80 bits of data
are encoded on said identification tag.
7. The system as recited in claim 4 wherein nineteen of said groups
are located on said substrate.
8. The system as recited in claim 7 wherein up to 112 bits of data
are encoded on said identification tag.
9. The system as recited in claim 3 wherein said group is comprised
of 21 of said locations having two of said reflectors arranged
therein separated by a minimum of ten reflector locations.
10. The system as recited in claim 3 wherein said group is
comprised of 16 of said locations having a single reflector
arranged therein.
11. A method of operating a system for determining an identifier on
a surface acoustic wave (SAW) identification tag, comprising:
causing a radio frequency (RF) interrogation signal to be generated
that excites a transducer located on a piezoelectric substrate into
producing an initial acoustic pulse that reflects off of a
plurality of reflectors arranged according to time and phase
position on said substrate to yield response acoustic pulses, said
transducer generating an RF response signal from said response
acoustic pulses; detecting said RF response signal and decoding
said identifier from said RF response signal in view of predefined
time, phase and amplitude parameters.
12. The method as recited in claim 11 wherein said RF interrogation
signal has a carrier frequency of about 2.44 GHz.
13. The method as recited in claim 11 further comprising a group of
reflector locations on said substrate.
14. The method as recited in claim 13 wherein a plurality of said
groups are located on said substrate.
15. The method as recited in claim 14 wherein fourteen of said
groups are located on said substrate.
16. The method as recited in claim 15 wherein up to 80 bits of data
are encoded on said identification tag.
17. The method as recited in claim 14 wherein nineteen of said
groups are located on said substrate.
18. The method as recited in claim 17 wherein up to 112 bits of
data are encoded on said identification tag.
19. The method as recited in claim 13 wherein said group is
comprised of 21 of said locations having two of said reflectors
arranged therein separated by a minimum of ten reflector
locations.
20. The method as recited in claim 13 wherein said group is
comprised of 16 of said locations having a single reflector
arranged therein.
Description
TECHNICAL FIELD OF THE INVENTION
The present invention is directed, in general, to a system for
determining an identifier on an identification tag and, more
specifically, to a transfer function system for determining an
identifier on a surface acoustic wave (SAW) identification tag.
BACKGROUND OF THE INVENTION
To address and overcome inherent existing limitations in prior art
radio frequency identification (RFID) tags with respect to cost,
data capacity and reliable range, a new RFID tag technology has
been developed. This new technology utilizes surface acoustic wave
(SAW) devices as identification tags and is described in detail in
U.S. patent application Ser. No. 10/066,173, entitled "Surface
Acoustic Wave Identification Tag Having Enhanced Data Content and
Methods of Operation and Manufacture Thereof," Harhnann, Clinton S.
("Hartmann One"), commonly assigned with the invention and
incorporated herein by reference. The principles used to encode
data on SAW tags involving simultaneous phase and time shift
modulation is described in detail in U.S. patent application Ser.
No. 10/062,833, entitled "Modulation by Phase and Time Shift Keying
and Method of Using the Same," Hartmann, Clinton S. (Hartmann-Two),
also commonly assigned with the invention and incorporated herein
by this reference. The principles used to encode data by combining
multi-pulse per group modulation with simultaneous phase and time
shift modulation is described in detail in U.S. patent application
Ser. No. 10/062,894, entitled "Modulation by Combined Multi-pulse
per Group with Simultaneous Phase and Time Shift Keying and Method
of Using the Same," Hartmann, Clinton S. (Hartrnann-Three), also
commonly assigned with the invention and incorporated herein by
reference. Additional pertinent information regarding SAW
identification tags and SAW identification tag readers is set forth
in detail in U.S. patent application Ser. No. 10/066,249, entitled
"Reader for a High Information Capacity Saw Identification Tag and
Method of Use Thereof," Hartmann, Clinton S. ("Hartmann Four"),
again commonly assigned with the invention and incorporated herein
by reference.
An interrogated RFID tag reflects or retransmits a radio signal in
response to an interrogation signal. The returned or reply signal
contains the data that, when decoded, identifies the tag and any
object with which the tag is associated. A SAW device used as an
identification tag can be encoded with a large amount of data. When
encoded with 64 or 96 bits of data, in accordance with certain
electronic product code (EPC) specifications, if such tags are to
be useful, a reliable system and procedure to accurately identify
the tag from a distance is required.
The problem can be best understood in the context of a user that
has a large number of objects, each with its own unique
identification tag. In order to identify a specific object among
the large number of objects, the user will send an interrogation
signal to be simultaneously received by a tag on each of the
objects. When each responds to the interrogation signal, there will
be a large quantity of data from which the signal from a single tag
must be isolated and identified. Thus, it is important for that SAW
tags be encoded in a manner that tags can be readily distinguished
from one another. A system is needed that can be used to encode SAW
tags with unique data that can readily be distinguished from the
data encoded on other SAW tags.
Accordingly, what is needed in the art is a reliable system for
determining the unique identifier encoded on a SAW identification
tag that can be readily decoded to identify the object with which
it is associated.
SUMMARY OF THE INVENTION
To address the above-discussed deficiencies of the prior art, the
present invention provides a transfer function system for
determining an identifier on a surface acoustic wave (SAW)
identification tag and a method of operating the same. In one
embodiment the system provides for (1) generating a radio frequency
(RF) interrogation signal that causes a transducer located on a
piezoelectric substrate to produce an initial acoustic pulse that
reflects off of a plurality of reflectors arranged according to
time and phase position on the substrate to yield response acoustic
pulses, the transducer generating an RF response signal from the
response acoustic pulses; and (2) determining the identifier by
decoding the RF response signal in view of predefined time, phase
and amplitude parameters.
The present invention thus provides a system for determining the
unique identifier encoded on a SAW identification tag. The system
takes advantage of certain known characteristics of SAW tags, which
are passive devices, to produce characteristically predictable
responses when excited by an interrogation signal. Because the
interrogation signal includes certain predetermined characteristics
that will be affected predictably by reflectors located on the SAW
tag, an analysis of this reflected response signal reveals the SAW
tag's configuration. That is, the response signal includes SAW tag
characteristics that are transferred to the interrogation signal.
Thus, because the characteristics of the interrogation pulse are
known, the possible SAW tag responses to an interrogation pulse are
also known; thus permitting a specific SAW tag to be identified
based on a transfer of information to the interrogation signal by
the SAW tag.
In one embodiment of the invention, the RF interrogation signal has
a carrier frequency of about 2.44 GHz. Of course, any other carrier
frequency can be used and still be within the intended scope of the
present invention. In a particularly useful and versatile
embodiment, the system provides for a group of reflector locations
on the substrate. One aspect of this embodiment provides for a
plurality of such groups to be located on the substrate. Yet
another aspect provides for a group to be made up of 21 reflector
locations with two reflectors arranged therein separated from each
other by a minimum of ten reflector locations. In still another
aspect, the group is comprised of 16 reflector slot locations with
a single reflector arranged therein.
In one embodiment of the system, fourteen groups of reflector
locations are located on the substrate. A useful feature of this
embodiment is that up to 80 bits of data can be encoded on the
identification tag. In another embodiment of the invention,
nineteen groups are located on the substrate, permitting up to 112
bits of data to be encoded on the identification tag.
The foregoing has outlined preferred and alternative features of
the present invention so that those skilled in the art may better
understand the detailed description of the invention that follows.
Additional features of the invention will be described hereinafter
that form the subject of the claims of the invention. Those skilled
in the art should appreciate that they can readily use the
disclosed conception and specific embodiment as a basis for
designing or modifying other structures for carrying out the same
purposes of the present invention. Those skilled in the art should
also realize that such equivalent constructions do not depart from
the spirit and scope of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
For a more complete understanding of the present invention,
reference is now made to the following descriptions taken in
conjunction with the accompanying drawings, in which:
FIG. 1 illustrates a surface acoustic wave (SAW) tag typical of a
type that can be used as a radio frequency identification (RFID)
tag;
FIG. 2 illustrates a representative layout of an embodiment of a
SAW tag utilizing fourteen groups of reflector locations on the
substrate to encode up to 80 bits of data; and
FIG. 3 illustrates a representative layout of an embodiment of a
SAW tag utilizing nineteen groups of reflector locations on the
substrate to encode up to 112 bits of data.
DETAILED DESCRIPTION
Referring initially to FIG. 1, illustrated is a surface acoustic
wave (SAW) tag 100 typical of the type that can be used as a radio
frequency identification (RFID) tag. The illustrated embodiment
provides for a reader antenna 105 that transmits a radio frequency
(RF) interrogation signal 110. The RF signal 110 is received by an
antenna 115 on the tag 100 and excites a transducer 120 located on
a piezoelectric substrate 130 so that it produces an initial
acoustic pulse 140. As the initial acoustic pulse 140 moves down
the surface 135 of the substrate 130, it encounters reflectors 150
located thereon, causing a reflection of a portion of the initial
acoustic pulse 140. This reflected pulse is called a response
acoustic pulse 160 herein.
A feature of the illustrated embodiment is that a plurality of
reflectors 150 are arranged on the substrate 130 according to time
and phase position to yield a plurality of response acoustic pulses
160. When the transducer 120 receives these response acoustic
pulses 160, an RF response signal 170 is generated that is
transmitted through the antenna 115 to be detected by a reader
antenna 105. The reader (not illustrated) then utilizes the system
described herein to determine the identifier in view of predefined
time, phase and amplitude parameters detected in the response
acoustic pulses 160.
The present system thus defines an identifier encoded on a SAW tag
100 by its "transfer function"; that is, the signal received is
determined by the SAW tag's 100 impulse response to the
interrogation signal. Defining a SAW tag 100 by its transfer
function approach is an appropriate methodology because SAW tags
100 are passive devices that react to and reflect energy that is
derived from the impinging signal. Because identifier encoded on
the SAW tag 100 is unique, the response signal 170 will also be
unique and the data encoded thereon can be uniquely determined
based on the transfer function of the SAW tag 100. Thus, it is the
transfer of SAW tag 100 features to the interrogation signal 110,
which features appear in the RF response signal 170 pattern that
defines the code embedded on a SAW identification tag 100.
The efficiency of using a transfer function methodology to define a
SAW tag 100 response is evident when contrasted with the more usual
"air interface" methodology for defining a signal response. With
air interface methodology a signal is defined without reference to
either the sending or responding devices, which is logical if both
such devices are active. In the case of SAW tags 100, however,
using air interface methodology to define signals would, by
necessity, also require an analysis of the SAW tag's 100 effect on
the interrogation signal 110 because a SAW tag is passive and only
responds to a signal stimulus. Thus, the usual signal processing
methods relying on an air interface approach for defining a SAW tag
100 would require the transfer function to be embedded in the
definition of the interrogation signal 110 as well as the
concomitant response signal 170.
An air interface methodology would be inefficient in describing the
specification of a SAW tag 100 because it would serve to
over-specify the essential requirements being used for
identification purposes. Specifying a SAW tag 100 by its transfer
function permits the use of a variety of approaches in designing
SAW tag readers because there is no limitation on the type of
interrogation signal 110 that can be used with the only real
constraint being that the interrogation signal 110 must meet the
appropriate governmental mandated emission requirements. Within
this constraint, any signal can be used that will produce a
response signal 170 with sufficient information to detect the
reflection pattern embedded in the SAW tag 100. Interrogation
signals such as, for example, (i) individual narrow pulses (e.g.
impulses); (ii) spectrum measurements for detecting the returning
amplitude and phase of multiple, individual tones; (iii) swept
frequency (e.g. chirp) signals; and (iv) coded (e.g. direct
sequence) spread spectrum signals can all be used as interrogation
signals.
The present invention thus provides a system for determining the
unique identifier encoded on a SAW identification tag 100 that
takes advantage of known characteristics of the SAW tag 100 to
produce a characteristically predictable response when excited by a
variety of types of interrogation signals. Because response
characteristics are affected predictably by reflectors 150 located
on the surface 135 of the SAW tag 100, it is an analysis of this
reflected response signal 170 that reveals the configuration unique
to the interrogated SAW tag 100. In one embodiment of the invention
the RF interrogation signal 110 utilizes a carrier frequency of
about 2.44 GHz. Of course, any other carrier frequency can be used
and still be within the intended scope of the present invention
provided the SAW tag 100 characteristics are described in terms of
the SAW tag 100 transfer function when any such other carrier
frequency is used.
In one embodiment of the present invention, transfer function of a
SAW tag 100 is defined in view of the externally observable values
of time, phase, amplitude and data order. As an externally
observable value, time is concerned with the time between RF
response signals 170 and implies that such time takes into
consideration the internal round-trip propagation travel time
between two reflectors 150 (or between a transducer 120 and
reflector 150). If the internal propagation time between two
reflectors 150 is t seconds, the externally observable value is 2t
seconds. In formulating the architecture of a SAW tag 100, time is
generally specified relative to the interrogation signal 110 pulse
duration. Thus, a value represented as 0.1T means a time duration
equal to one-tenth of the duration of the interrogation signal 110
pulse. Unless otherwise specified, it is generally understood that
time is measured from the center of a pulse.
In considering externally observable phase value, the internal
reflection phase and the phase shift in the interrogation signal
110, as propagated by the transducer 120 as the initial acoustic
pulse 140, reflected as the RF response signal 170 must be jointly
considered. Phase shift arising from internal propagation is
dependent on the carrier frequency for the interrogation signal
110, which, as noted above, is assumed to be 2.44 GHz (the middle
of the ISM band) for the purpose of this description. As will be
understood by those of ordinary skill in the pertinent art,
however, if a different carrier frequency is used as an
interrogation signal 110, the externally observable phase values
must be correspondingly adjusted.
For the purpose of defining an externally observable RF response
signal 170 in terms of the structural parameters of a SAW tag 100,
the amplitude of the RF response signal 170 is specified by
assuming the initial acoustic pulse 140 has an amplitude of 1.0 and
a comparative number being used for the reflected response acoustic
pulse 160. Any specified pulse amplitude must take into
consideration any internal aspects of propagation attenuation,
reflection coefficients, and transmission loss.
When using externally observable values to define transfer function
of a SAW tag 100, it is necessary to make certain assumptions
regarding how the data and data fields are presented in the signal.
For the purpose of this description, it is assumed that the least
significant bit (lsb) will be placed on the right and the most
significant bit (msb) on the left which implies that the order of
transmission of data will be the msb first. As will be understood
by those of ordinary skill in the pertinent art, different
assumptions on how data and data fields are presented in the signal
can be made and still be within the intended scope of the present
invention.
A particularly useful and versatile embodiment of the present
invention provides for the system to have a group 180 of reflector
locations 190 on the substrate 130. Thus, the SAW tag 100 can be
designed to produce an external observable value based on the
transfer function of the SAW tag 100 as defined by the location of
reflectors 150 in a group 180 of predetermined reflector locations
190. In one embodiment of the present invention, the system
provides for an encoding algorithm of a group 180 of 21 reflector
locations 190 having two reflectors 150 arranged therein separated
by a minimum of ten reflector locations 190. The two reflectors 150
are separated by a minimum of 10 reflector locations 190 to
preclude significant overlap between response acoustic pulses 140.
Additional pulse separation can be achieved by increasing the
number of reflector locations 190 between pulses. As here in after
explained, this "2-of-21"embodiment provides the capability for
encoding multiple bits of data that can be decoded with a small
number of response acoustic pulses 140. Although other encoding
algorithms may allow more data to b e encoded in less space, the
2-of-21 system provides the advantage of encoding simplicity,
flexibility and the possibility of an increased uniformity in
amplitude of response acoustic pulses 140.
As an aid in determining and selecting reflector locations 190 from
a group 180 of closely spaced locations 190 on a SAW tag 100,
adjacent locations 190 provide for different phase values for a
reflected response acoustic pulse 140. Table 1 defines the
reflector locations 190 and the relative reflection phases assigned
for an embodiment encoded using the 2-of-21 system.
TABLE 1 Twenty-one Position Encoding Group 1 0 0.0 2 -64 0.1 3 -128
0.2 4 -192 0.3 5 -256 0.4 6 -320 0.5 7 -24 0.6 8 -88 0.7 9 -152 0.8
10 -216 0.9 11 -280 1.0 12 -344 1.1 13 -48 1.2 14 -112 1.3 15 -176
1.4 16 -240 1.5 17 -304 1.6 18 -8 1.7 19 -72 1.8 20 -136 1.9 21
-200 2.0
This embodiment provides for reflector locations 190 nominally
spaced at intervals equal to ten per cent of the width of the
interrogating pulse expressed as a unit of time. As those of
ordinary skill in the pertinent art will understand, other phase
positions, times, and reflector locations 190 can be used and still
be within the intended scope of the present invention.
With twenty-one potential locations 190 and a minimum spacing of
ten locations 190 between reflectors 150, there are 66 data
combinations that can be decoded with two response acoustic pulses
160. As shown in the following Table 2, sixty-four of these
combinations can be used to represent 6 bits of information.
TABLE 2 Two of Twenty-one Data Encoding Code # Code Pulse 1 Pulse 2
1 000000 1 11 2 000001 1 12 3 000011 1 13 4 000010 1 14 5 000110 1
15 6 000111 1 16 7 000101 1 17 8 000100 1 18 9 001100 1 19 10
001101 1 20 11 001111 1 21 12 001110 2 21 13 001010 2 20 14 001011
2 19 15 001001 2 18 16 001000 2 17 17 011000 2 16 18 011001 2 15 19
011011 2 14 20 011010 2 13 21 011110 2 12 22 011111 3 13 23 011101
3 14 24 011100 3 15 25 010100 3 16 26 010101 3 17 27 010111 3 18 28
010110 3 19 29 010010 3 20 30 010011 3 21 31 010001 4 21 32 010000
4 20 33 110000 4 19 34 110001 4 18 35 110011 4 17 36 110010 4 16 37
110110 4 15 38 110111 4 14 39 110101 5 15 40 110100 5 16 41 111100
5 17 42 111101 5 18 43 111111 5 19 44 111110 5 20 45 111010 5 21 46
111011 6 21 47 111001 6 20 48 111000 6 19 49 101000 6 18 50 101001
6 17 51 101011 6 16 52 101010 7 17 53 101110 7 18 54 101111 7 19 55
101101 7 20 56 101100 7 21 57 100100 8 21 58 100101 8 20 59 100111
8 19 60 100110 8 18 61 100010 9 19 62 100011 9 20 63 100001 9 21 64
100000 10 21 65 10 20 66 11 21
Another useful encoding algorithm is the 1-of-16 encoding format.
This provides for a single reflector 140 located in one of sixteen
reflector locations 190 and can be used to encode four bits of
data. Table 3 lists the phase values for each location 190 and the
code that each such location 190 represents for one embodiment of
this algorithm.
TABLE 3 Sixteen Position Encoding Group Phase Relative Pulse Value
Delay Position (degrees) (T) Code 1 0 0.0 0000 2 -64 0.1 0001 3
-128 0.2 0011 4 -192 0.3 0010 5 -256 0.4 0110 6 -320 0.5 0111 7 -24
0.6 0101 8 -88 0.7 0100 9 -152 0.8 1100 10 -216 0.9 1101 11 -280
1.0 1111 12 -344 1.1 1110 13 -48 1.2 1010 14 -112 1.3 1011 15 -176
1.4 1001 16 -240 1.5 1000
Turning now to FIG. 2, illustrated is a representative layout 200
of an embodiment of a SAW tag utilizing fourteen groups 210 of
reflector locations on the substrate to encode up to 80 bits of
data. In the illustrated embodiment, the first twelve groups 210
use 2-of-21 encoding while the last two groups 210 use 1-of-16.
Thus the fourteen groups 210 provide for 80 bits of data encoding
(12.times.6 plus 2.times.4=80). As will be understood by those of
ordinary skill in the pertinent art, the 80 bits of data can be
structured to provide for 64 bits of code (as may be required by
relevant electronic product code (EPC) specifications), leaving 16
bits of data to be used for error checking, frame and phase
synchronization, and SAW tag version information.
In the illustrated layout 200, a preamble 220 precedes data groups
230 and provides for functions such as frame and phase
synchronization as well as providing data space for SAW tag version
information. The fourteen groups 210 are separated by time values
215 (labeled t.sub.1 through t.sub.14). Each time value 215
interval represents the time between the center of the last
reflector position in one group 210 to the center of the first
reflector position of the next group 210. Also shown is a time
delay value 216 (labeled as Delay0), before a reflector can produce
a response acoustic pulse to an interrogation signal. This delay
provides separation between the SAW tag response and other
relatively high energy reflections of the interrogation signal. It
also provides for a method to distinguish between separate classes
of SAW tags. For example a warehouse user of SAW tags may identify
three primary applications, such as palettes, cases, and items. In
the interest of maximizing the ability to detect a tag in one class
in the presence of one or more tags from another class, differing
amounts of initial delay can be established. Other methods of
distinguishing SAW tag versions include preamble 220 formatting and
error checking. In the case of preamble formatting, different
versions can be defined by modifying preamble pulse separations,
phase encoding, or combinations thereof. In the case of error
checking, if a valid check sum is not obtained while assuming one
version, the return signal can be processed using other assumed
versions until a valid check sum occurs.
An advantageous method for distinguishing between SAW
identification tags is to scramble the data and data fields on the
SAW identification tags. When a series of SAW identification tags
are manufactured using sequential coded numbers, the difference in
response signals returned by two sequentially numbered SAW tags
would be minimal. To facilitate the ability to distinguish between
similarly coded SAW tags, the data encoded thereon can be scrambled
to create widely different pulse patterns for each code on a SAW
tag without changing such data such as the header, object, and msb.
The use of differing pulse patterns will facilitate identifying
individual SAW tags in an ensemble of SAW tags. Further
facilitation can be achieved by increasing the scrambled pulse
separation.
In order to illustrate the scrambling concept, assume a series of
SAW tags are produced having encoded thereon 64-bits of data and
16-bits of error correction for an aggregate payload of 80 bits.
Before the 80-bit payload data is encoded into all fields except
B0, B1,B2, B3 and EC, the other fields are scrambled by bit-by-bit
"exclusive OR" of these fields with the twelve bits of B0 and B1.
Multiple versions of scrambling codes are created by end-around
shifting of B0 and B1. A designation of SN0-i indicates an
end-around shift to the left of i bit positions of B0 (the lsb of
B0 appears i bit positions to the left). Similarly, a designation
of B1-i indicates an end-around shift to the left of i bit
positions of B1. Table 4 identifies the scrambled fields and the
particular shift value of B0 or B1 used to scramble the respective
fields. The output codes of the scrambling processes are designated
in sub-fields of a 40-bit codeword S. Sub-fields S0 to S5 are six
bits in length while sub-field S6 is four bits in length which is
produced by exclusive "OR"ing B10 with the four lsb of B0.
TABLE 4 64-Bit Scrambling Process ##STR1##
The SAW tag codes are scrambled to create widely different pulse
patterns for each code in a series of closely related tags despite
the fact that certain fields will be the same. The use of differing
pulse patterns facilitates identifying the individual SAW tags in
an ensemble of such tags. The B0 and B1 fields were selected to use
for the scrambling codes in the illustrated example, because it is
assumed that they will change from one ensemble of SAW tags to
another. To avoid using the same scrambling code in multiple
fields, the code is shifted to produce different codes in S0
through S6. Thus, even though unscrambled fields might be
identical, the scrambled codes will not be identical.
The SAW tag response signal format of a 64-bit data code is shown
in Table 5. The generic fields of the 64-bit code shown in Table 5
are transmitted in the order B0 to B3 followed by the scrambled
fields S0 to S6. The transmission of the 16 Error Check bits will
follow the 64-bit data field.
TABLE 5 64-Bit Unscrambling ##STR2##
Assuming B0 and B1 are received correctly (as subsequently verified
by the error check), the scrambled fields can be unscrambled by
reversing the process used in scrambling, as illustrated in Table
6.
TABLE 6 64-Bit Unscrambling Process ##STR3##
Turning now to FIG. 3, illustrated is a representative layout 300
of an embodiment of a SAW tag utilizing nineteen groups 210 of
reflector locations to encode up to 112 bits of data. The
illustrated embodiment provides for the first twelve groups 210 to
use 2-of-21 encoding while the last two groups 210 use 1-of-16.
Thus the fourteen groups 210 provide for 80 bits of data
encoding(12.times.6 plus 2.times.4=80). Eighteen groups of 2-of-21
encoding plus one group of 1-of-16 encoding provide the 112 bits of
data, which can be encoding as 96 bits of EPC code and 16 bits of
error check. Encoding this embodiment is fundamentally the same as
encoding the embodiment illustrated in FIG. 2.
Although the present invention has been described in detail, those
skilled in the art should understand that they can make various
changes, substitutions and alterations herein without departing
from the spirit and scope of the invention in its broadest
form.
* * * * *